1. Field of the Invention
The present invention relates to an improvement in a toner supply controlling device in a reproducing apparatus and the like developing an electrostatic latent image by using a developer which is composed of a toner and a carrier, and in the broadest sense it relates to an improvement in a supply controlling device employed to replace the toner consumed in a continuous operation of recording picture images in an electrostatic recording apparatus.
2. Description of the Prior Art
Recently, wide use has been made of electrostatic recording apparatuses, such as reproducing apparatuses, in which a dry developing device using a powdery or granular developer composed of a toner and a carrier is incorporated.
It is known that the density of the toner in the developer has an important effect on the quality of the image and, in particular, on the density of the image, when the development is performed by using a developer which is a mixture of a toner and a carrier. Therefore, various toner supply controlling devices which control the rate of supply of toner to the developer have been proposed and put in practical use.
One example of such a conventional toner supply controlling devices is a device provided with a means for measuring the density of a developed image and a toner supply controlling means controlling the supply of toner on the basis of measurement information obtained therefrom, which is described in the Official Gazette concerning the Japanese Patent Publication No. 16199/1968. This device controls the supply of toner, primarily, so that the density of the developed image, i.e. the development density, is constant, and therefore it is an excellent control device in that it ensures the density of a recorded image more directly than in other devices. Another toner supply controlling device is known which controls the supply of toner so that the density of the toner in the developer is constant by detecting the density by a measurement of the magnetic permeability and color density of the developer. Such a device, however, can only give an indirect assurance of the stability of the development density, and the density of the recorded image closely related to it, although it is preferable for fixing the density of the toner in the developer. Accordingly, in order to obtain a clearly recorded image, a toner supply controlling device which controls according to the development density is preferable.
However, the toner supply controlling device of either of these systems contains elements that could generate errors due to various disturbance factors, and thereby the obstacles described below could occur. Such errors could be caused by errors in the measurement information from the measuring means, or by a malfunction in the electric system of the control device. The following is an explanation of one of these errors which could happen when the supply of toner is controlled according to information on the measured development density. In the device wherein the supply of toner is controlled according to information on the development density, the development density is not only dependent on the density of the toner in the developer, even when the device is designed to determine the development density from a comparison with a reference density plate, and the development density sometimes decreases, although the density of the toner in the developer is correct, because fluctuations of the potential of the electrostatic latent image become large or because the toner is charged far too high (e.g. 40-50 .mu.C/g) due to frictional electricity generated between the toner and the carrier, when several hundred sheets of copies are prepared in succession, and when the reproduction is conducted successively under such unusual ambient conditions as a temperature of 5 .degree. C., and a relative humidity of less than 20%. Since the means for controlling the supply of toner must conduct the supply of toner continuously to ensure the development density, the density of the toner in the developer is increased too much, which results in the scattering a large quantity of toner from the developing device. The scattered toner often adheres to the lens, mirror, etc., of the exposure device or to the electric charger, the transfer device, etc., or reduces the clarity of the recorded image.